1. Field of the Invention
This invention relates to a method for assessing the tendency of hydrocarbon-based liquid fuels to form insoluble products during thermal stress encountered in aircraft fuel systems. More specifically, it relates to an accelerated method for assessing such insoluble product formation which avoids subjective human judgment, such as the color of a test specimen, and relies on an objective, gravimetric measurement.
2. Description of the Prior Art
Hydrocarbon-based fuels such as kerosene, diesel fuel, and jet fuel tend to form fuel-insoluble solid products during thermal stress, defined as exposure to elevated temperature for short or prolonged periods of time. Liquid fuels are normally exposed to such thermal stress in aircraft fuel systems. Fuel-insoluble solids may clog fuel filters and fuel atomizing nozzles in engines, coat heat exchanger surfaces, and obstruct close-tolerance fuel control mechanisms, thereby causing operating problems.
The formation of fuel-insoluble solids in hydrocarbon-based liquid fuels is a function of the presence in the fuel of certain chemical components which at present have not been identified. Empirical test methods are therefore needed for assessing the tendency of any given fuel to form fuel-insoluble solids during thermal stress in an aircraft fuel system. As a practical matter, such tests must be completed within about 24 hours. There is a need therefore for an accelerated test method for assessing the formation of fuel-insoluble solids in a fuel which yields results within a few hours.
U.S. Pat. No. 3,647,404 describes a thermally stable turbine or jet fuel composition comprising a mineral oil distillate and a low concentration of an additive inhibiting the formation of fuel-insoluble solids in turbine or jet fuels.